1. Field of the Invention
This invention relates to synchronizing audio and video tape players.
2. Description of the Prior Art
There are various applications where professional video equipment users, such as video production companies and/or broadcasters, may wish to synchronize audio and video tape players, for example to mix audio and video together in synchronism on a new tape or to broadcast the audio and video together in synchronism. Examples of such applications are in post-production, where video and audio may be recorded at different times and have to be brought together, or for bringing in special effects recorded on a library tape. Another example is where it is desired to transmit programs in several languages and the video tape does not have enough tracks for all of the languages, whereby there is a need to play an audio tape in parallel with the video tape.
It is known to facilitate such synchronization of audio and video tape players by the use of time code signals that are recorded, for example on so-called time code tracks, on the tapes played by the audio and video tape players. The time code signal comprises a sequence of digital words, each of which identifies a respective video frame. That is, each video frame on the video tape, or each portion of a sound track on the audio tape to be associated with a video frame, is identified by a time code. To achieve synchronization of the tapes on playback, the time codes are used to position the tapes in a desired temporal relationship and frame pulse signals derived from the time codes are then locked together to maintain synchronism between the tapes.
The foregoing will now be explained in more detail with reference to FIG. 1 of the accompanying drawings, which shows a previously proposed apparatus for synchronizing audio and video tape players. The apparatus shown in FIG. 1 comprises a video tape recorder (VTR) 10 and an audio tape recorder (ATR) 12. (For the avoidance of confusion, it should be noted that, although the items 10 and 12 are recorders, they are used only as players in the present application. It should further be noted that, although the item 12 is described as being an ATR, it may in fact be a VTR and the tape played thereby may be a video tape also having audio information recorded thereon, though in the present application only the audio information is needed.)
The VTR 10 and ATR 12 produce video and audio output signals, respectively, these signals not being shown in FIG. 1. These signals may, for example, be mixed together and recorded on another tape and/or broadcast simultaneously, as explained above.
The tapes played on the VTR 10 and ATR 12 include time code signals which are outputted via lines 14 and 16, respectively, to respective time code readers (TCRs) 18 and 20, respectively. Each time code signal comprises a sequence of digital words each identifying an associated video frame. As shown in FIG. 2 of the accompanying drawings, each word of the time code signal has a length of 80 bits. The 80 bits comprise 32 bits which indicate time (in hours, minutes and seconds on a 24 hour clock basis) and the frame number, 32 bits which the user can employ for various purposes, and 16 synchronization bits. Rather than being, as shown, arranged in sequence, the bits of the two groups of 32 bits are in fact interleaved. The detailed format of the bits is known in the art and full details can be obtained from relevant standards published by the SMPTE (Society of Motion Picture and Television Engineers) and the EBU (European Broadcasting Union).
The TCRs 18, 20 may be proprietary integrated circuits, for example CX 7912A integrated circuits marketed by Sony Corporation. In a manner known per se, each TCR 18, 20 decodes each word of the time code signal to produce information representing the time and frame number on lines 22, 24, respectively, which information can, for example, be viewed. Also in a manner known per se, each TCR 18, 20 produces on a line 26, 28, respectively, a frame pulse signal shown in FIG. 3 of the accompanying drawings. As shown in FIG. 3, each frame pulse signal is a square wave having a period equal to the frame period, namely (for example) 40 ms in a 25 frame per second system. Further, and again in a manner known per se, the TCRs 18, 20 recover from the time code signals, as well as the frame pulse signals, data clock pulse signals which are used internally of the TCRs to decode the data contained in the words of the time code signals. The frequency of the data clock pulse signals is 80 times that of the frame pulse signals, being that of the bits making up each word of the time code signal. The data clock pulses generated and utilized internally of the TCRs 18, 20 are made available to the exterior of the TCRs. More specifically, as shown, a video data clock pulse signal VDCLK is made available on a line 29 and an audio data clock signal ADCLK is made available on a line 30, the lines 29, 30 being connected to respective inputs of a dropout detector 31. The signals VDCLK and ADCLK are of the same form and are both as shown in FIG. 5 of the accompanying drawings at (A).
The lines 26, 28 carrying the video and audio frame pulse signals, respectively, are connected to respective inputs of a phase detector 32. An error signal produced on an output of the phase detector 32 is connected via a low pass filter 33 and a switch 34 to a line 35 connected to an input of the ATR 12 that controls the reproduction speed of the ATR 12 in accordance with the level of the signal applied to such input.
An output of a fixed drive 36 is connected to the switch 34. The switch 34 is controlled by a control microprocessor 37 whereby a fixed level from the fixed drive 36 can be applied to the ATR 12, in place of the filtered error signal, by changing over the switch from the illustrated condition.
A dropout detect signal from the dropout monitor 31 is connected by a line 38 to the phase detector 32 and to the microprocessor 37.
The apparatus shown in FIG. 1 operates in the following manner. The VTR 10 and ATR 12 are required to reproduce or play respective tapes such that there is a desired temporal relationship between the video and audio material recorded on the respective tapes, that is to say such that there is a desired difference (which in some cases might be zero) in terms of hours, minutes, seconds and frame number between the information being reproduced by the VTR 10 and ATR 12 at any one time. Such relationship is initially established by running one or both of the VTR 10 and ATR 12 until, for example by viewing the time and frame numbers represented by the information on the lines 22, 24, the two tapes are within one frame of the desired temporal relationship. During this initial setting up, the locking together or synchronization (described below) of the two tapes will be inhibited. Once the temporal relationship has been established to the nearest frame, the exact temporal relationship is achieved and thereafter maintained by the circuit of FIG. 1 as will now be described.
When the switch 34 is in the illustrated condition, the phase detector 32, ATR 12 and TCR 20 form a phase locked loop (PLL) that controls the speed of reproduction of the ATR 12 to conform with that of the VTR 10. Thus, the phase detector 32 detects the leading edges of the video and audio frame pulse signals applied thereto on the lines 26 and 28, respectively, and, in response to any difference in the timing thereof, produces the above-mentioned error signal which acts on the loop to control the speed of the ATR 12 until the leading edges of the two frame signals are brought into synchronization. Thereafter, the PLL acts indefinitely to adjust the speed of the ATR 12 as necessary to keep the leading edges of the two frame pulse signals in synchronization whereby the audio and video are reproduced in synchronism.
During the operation of the apparatus of FIG. 1, a problem will arise if, for example due to poor recording, one or both of the time code signals should be lost. Consider, for example, what happens if the video time code signal (and therefore the video frame pulse signal) disappears. In the absence of any measure to the contrary, the PLL will assume that the VTR has stopped and will therefore act to stop the ATR 12. Naturally, this is not desired, since the VTR 10 is still in fact running. The dropout monitor 31 prevents this happening. The dropout monitor 31 monitors for the presence of both time code signals (indirectly so, by monitoring for the presence of the data clock pulse signals VDCLK and ADCLK) and is responsive to either or both of these signals disappearing to produce the dropout detect signal on the line 38. This gives rise to the following. Firstly, the phase detector 32 is inhibited by the dropout detect signal, which "freezes" the drive to the ATR 12. Secondly, the microprocessor 37 is responsive to the dropout detect signal to change over the switch 34. This breaks the PLL and causes the fixed level from the fixed drive 36 to be applied to the ATR 12 (instead of the filtered error signal) via the line 35 to cause the ATR to continue running at a fixed, nominal speed. While this prevents the ATR 12 being stopped in the event of either of the frame pulse signals disappearing, the ATR is not being synchronized while both frame pulse signals are not present and will gradually drift out of synchronization with the VTR 10, a drift of around one frame per minute being typical. This leads to the following problem. When the disappearance of the one or more frame pulse signals ends, the dropout detect signal produced by the dropout monitor 31 disappears, as a result of which the phase detector 32 is reenabled and the microprocessor 37 changes over the switch 34 so that it readopts its illustrated condition. Thus, the PLL comes back into operation. When it does so, it restores synchronization abruptly. Thus, if the accumulated drift or error is a significant proportion of a frame, the relocking operation will be clearly audible: there will be an abrupt pitch change until the frame pulse signals are again phase locked together.
An object of the invention is to provide a synchronizing method and apparatus of the above general kind in which the relocking operation after loss of one or both of the time code signals is less abrupt and therefore less noticeable.